An electret microphone of the kind intended here is described, e.g. in the Swedish patent No. 8302197-2. It includes a casing made from a upper and lower casing halves and it contains a metal electrode, a metallized electret film and an amplifying element, which are enclosed and located by the upper and the lower casing halves and their fastenings. The problem is to keep the electret film in place in the microphone with a given clamping force and at a given distance from the rear metallic electrode. In the known electret microphone, this has been solved by placing a spacer washer of insulating material between the film and the rear electrode.